Sen. Rob Portman could help put Mitt Romney over the top in Ohio, a potential boost that has nothing to do with him joining the Republican ticket as Romney’s running mate.

Since launching his first statewide campaign three years ago, Portman has built a strong political network throughout Ohio, including in the rural counties rich in conservative voters that could be crucial to Romney’s success this fall. Perhaps more importantly for Romney in his bid to oust President Barack Obama, Portman has credibility with a broad cross section of the party sufficient to activate this network on behalf of others.

“Along the Ohio River, there’s a lot of people that will walk on hot coals for Rob Portman,” Brown County GOP Chairman Paul Hall said. “If he comes in and says, ‘We need to do this,’ it’s going to happen because Rob needs them to do it.”

Republicans have never won the White House without Ohio’s electoral votes, which total 18 this year. That fact and the influence the Buckeye State’s rural counties could have on the presidential contest are not lost on team Romney. The former Massachusetts governor’s first bus tour of the general election campaign cut a rare path through small towns and rural outposts in six battleground Rust Belt states, including Ohio.

Republican officials in rural Ohio counties confirmed that the Romney campaign has communicated with them regularly since the GOP primary concluded, including through in-person staff visits and invitations to participate in conference calls. These officials said they have not received this much attention since President George W. Bush’s 2004 re-election campaign. In 2008, they saw Obama in their area more than they heard from or saw Sen. John McCain (R-Ariz.).

Romney’s outreach to rural Ohio is vital given his weak performance there throughout the contentious Republican primary. But his prospects against Obama depend on much more than rural turnout. Romney will need to exhibit strength in suburban and exurban battlegrounds near Cincinnati in the southwest, around Cleveland in the northeast, and surrounding Columbus in central Ohio. These are high-population regions loaded with swing voters and independents.

“Here’s a guy from Cincinnati who had never run statewide, and he did very well in northeast Ohio,” said Barry Bennett, an Ohio GOP consultant who has advised Portman. “He now has a network of folks from all over the state.”

Portman’s office declined to comment.

From 1993 to 2005, Portman represented a House district anchored by the Cincinnati suburbs that stretched into rural counties along the Ohio River. It is a GOP stronghold and a national stop on the Republican fundraising circuit. The Senator’s political coalition and donor network there have long been muscular.

It’s Portman’s ability to branch out across the state and forge ties with key Republican constituencies, including rank-and-file activists, tea party supporters and moneyed campaign contributors, that makes the former Bush administration official potentially so valuable to Romney.

Last week, Romney publicly thanked Portman during an Ohio campaign stop for helping him win March’s close Ohio presidential primary. Romney beat former Sen. Rick Santorum (Pa.) by only 1 point, and Portman’s Ohio backers said Romney’s compliment was more than the polite hyperbole that is typically offered by nominees as they barnstorm with their supporters.

“His endorsement carried a lot of weight,” Hamilton County GOP Chairman Alex Triantafilou said. “Rob manages to pull all facets of the party together. ... They all like him. At the end of the day, he’s a unifying force.”

In 2004, Bush won Ohio by 2 points, in part by ginning up turnout in rural counties that many presidential candidates had previously ignored. Four years later, Obama carried the state by 4.5 points. He campaigned in rural Ohio enough to make a measurable dent in McCain’s margin of victory in the region.

In fact, showing up can be half the battle in competing for what is now recognized as a difference-maker region for GOP presidential candidates. This is particularly the case if rural voters view the nominee suspiciously, as was the case with Romney in this year’s primary — and with Portman when he launched his 2010 Senate campaign. Hall, the Brown County GOP chairman, recalled his opposition to Portman dating back to the first of his seven House terms.

Portman was perceived as an establishment, suburban Cincinnati politician in tune with business interests — and he was. But the mild-mannered Republican rented a Chevrolet RV and traveled to every county in the state. He occasionally slept in the vehicle. At nearly every stop, he courted activists and voters without regard for a county’s electoral significance, his supporters noted approvingly. Portman won the Senate race by 18 points, a margin no doubt padded by the magnitude of the national GOP wave in 2010.

In Washington, D.C., Portman has a reputation for being wonky, workman-like and boring. Ohio Republicans describe him as personable and a good listener. He remembers the names of family members and their interests, and he never fails to ask how everyone is doing.

Republicans believe the Senator’s political network will work hard for Romney if Portman asks them to, even if the presumptive nominee does not select the Ohioan to be his running mate.

“He does his homework and knows how we live,” Adams County Commissioner Roger Rhonemus said. “A lot of the county folks and communities trust his judgment.”